Conventionally, a feedback system was developed in which an engine combustion condition was detected so as to determine the amount of engine control and the combustion condition was controlled in accordance with the amount of determined engine control. A problem in this conventional method is the unstable operation of the feedback system. As a result of studying the cause of this problem, it has been determined that in conventional feedback systems the detection of the engine combustion condition is effected in accordance with the change of engine crank angle and the sensor output is processed as a function of crank angle, thus making correct detection of the combustion conditions impossible.